RE: [PATCH] clk: baikal-t1: Using div64_ Ul replaces do_ Div() function

[David Laight]

From: Geert Uytterhoeven
> Sent: 24 July 2023 14:39
> 
> Hi Serge,
> 
> On Mon, Jul 24, 2023 at 3:13 PM Serge Semin <fancer.lancer@xxxxxxxxx> wrote:
> > On Mon, Jul 24, 2023 at 12:04:19PM +0200, Geert Uytterhoeven wrote:
> > > On Wed, Jun 14, 2023 at 8:07 AM <wuyonggang001@xxxxxxxxxx> wrote:
> > > > Fix the following coccicheck warning:
> > > >
> > > > drivers/clk/baikal-t1/ccu-pll.c:81:1-7: WARNING: do_div() does a
> > > > 64-by-32 division, please consider using div64_ul instead.
> > > >
> > > > Signed-off-by: Yonggang Wu <wuyonggang001@xxxxxxxxxx>
> > >
> > > Thanks for your patch, which is now commit b93d1331ea266dea
> > > ("clk: baikal-t1: Using div64_ Ul replaces do_ Div() function")
> > > in clk/clk-next.
> > >
> > > > b/drivers/clk/baikal-t1/ccu-pll.c
> > > > index 13ef28001439..d41735c6956a 100644
> > > > --- a/drivers/clk/baikal-t1/ccu-pll.c
> > > > +++ b/drivers/clk/baikal-t1/ccu-pll.c
> 
> > > > @@ -78,9 +78,9 @@ static inline unsigned long ccu_pll_calc_freq(unsigned
> > > > long ref_clk,
> > > >   {
> > > >       u64 tmp = ref_clk;
> > > >
> >
> > > > -    do_div(tmp, nr);
> > > > +    div64_ul(tmp, nr);
> > > >       tmp *= nf;
> > > > -    do_div(tmp, od);
> > > > +    div64_ul(tmp, od);
> > > >
> > > >       return tmp;
> > >
> > > Likewise.
> >
> > Right. This will also break the driver.
> >
> > > But as ref_clk is unsigned long, there is no need to use div64_ul()
> > > for the first division, and this can be simplified to:
> > >
> > >     u64 tmp = (u64)(ref_clk / nr) * nf;
> > >     return div64_ul(tmp, od);
> >
> > Absolutely right. My intention of using the do_div() anyway was for
> > the sake of the code unification.
> >
> > >
> > > To avoid loss of precision, it might be better to reverse the order
> > > of the division and multiplication:
> > >
> >
> > >     u64 tmp = (u64)ref_clk * nf / nr;
> >
> > Alas exactly this code will cause the compilation error on the 32-bit
> > platform:
> > ccu-pll.c:(.text+0x458): undefined reference to `__udivdi3'
> >
> > That's why I am using the do_div() here. I would have rather used the
> > div64_ul() instead as this patch suggests, but I haven't known about its
> > existence up to this moment.
> 
> Bummer, that was a silly mistake on my side...
> (Initially, I didn't write the cast to u64 there, as all of ref_clk, nf, and nr
>  are unsigned long.  Then I realized "ref_clk * nf" might overflow on
>  32-bit, thus requiring a 64-bit result. And I added the cast...)

But on 32bit the result is 'long'.
So it will overflow unless do_div() is also valid.

The analysis need to look at the domain of the values.
The warning and suggestion to use div64_ul() is pretty much always
wrong.

div64_ul() is going to be horribly slow on 32bit.
Also on 64bit Intel cpu the 128/64 divide takes twice as long as 64/32
even when the values are small.

	David

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